The present invention relates to an electrochromic display device (referred to as an ECD hereafter). More particularly, the present invention relates to a refreshing method of coloration degree in an ECD which is driven by transmitting a coloration electric charge kept by an electrode group in a coloration state to an electrode group in a bleaching state, by applying a voltage between the electrode group in the coloration state and the electrode group in the bleaching state (referred to as an electric charge transmission drive). The ECD driven by the electric charge transmission is excellent in uniformity in display color and a quick response. FIG. 1 shows a schematic illustration of an ECD panel for explaining the charge transmission drive.
In the ECD panel, transparent electrodes 2a and 2c are formed on a plane of a transparent substrate 1 by evaporating In.sub.2 O.sub.3 or the like, and films of electrochromic material 3a to 3c are formed on the transparent electrodes 2a to 2c in the configuration of display picture elements by evaporating WO.sub.3 or MoO.sub.3.
Though omitted in FIG. 1, the transparent electrode portion other than the display picture element pattern region is covered with an insulating film. The transparent electrodes 2a to 2c and the films of electrochromic material 3a to 3c are designated generally as display electrodes A to C. An electrolyte 7 is sealed within spacers 6 placed between the substrate 1 and a substrate 4 on which a counter electrode 5 formed by Au or the like is provided. The electrolyte is mixed with a white powder such as TiO.sub.2 to constitute a background of the display.
Referring to the circuit portion, coloration switches 8a to 8c respectively connect each of the transparent electrodes 2a to 2c to the cathode of a battery 12. Bleaching switches 9a to 9c, respectively connect each of the transparent electrodes 2a to 2c to the anode of the battery 12. A switch 10 for eliminating the coloration electric charge connects the counter electrode 5 to the cathode of the battery 12. A switch 11 for injecting the coloration electric charge connects the counter electrode 5 to the anode of the battery 12.
Now the operation of the ECD by the above described structure will be illustrated.
The coloration electric charge is first injected into the ECD panel. The electrode to which the coloration electric charge is injected initially is designated as the display electrode A. When the switch 8a and the switch 11 are closed, a current flows from the counter electrode 5 to the display electrode A, and the film of the electrochromic material 3a is deoxidized and colored. After the film of the electrochromic material 3a reaches the predetermined coloration degree, the switches 8a and 11 are opened. Then the electrochromic material 3a keeps the deoxidization state and the display memorizes the coloration state.
Referring next to the method of the electric charge transmission, the coloration electric charge kept in the display electrode A is transmitted to the display electrode C by closing the switches 9a and 8c and connecting the cathode of the battery 12 to the display electrode A and connecting the anode of the battery 12 to the display electrode C. At this time the coloration electric charge is released from the display electrode A, transmitted through the electrolyte 7 and injected to the display electrode C. As a result, the display electrode A changes from the coloration state to the bleaching state and the display electrode C assumes the coloration state and the display state changes.
The above noted electric charge transmission drive is disadvantageous in that the coloration degree deteriorates with an increase in the number of transmissions, since a slight transmission loss exists in the electric charge transmission.
FIG. 2 shows an experimental data of the coloration degree and the number of transmissions. The ordinate indicates a reflection rate of the display portion and the abscissa indicates the number of transmissions in FIG. 2. The white background is the standard of 100% reflection rate, and the more the coloration degree, the less the reflection rate. FIG. 2 teaches that the coloration degree is substantially constant until the number of transmissions is 10.sup.3 times, and thereafter the coloration degree abruptly deteriorates. Therefore a difference in the coloration degree, i.e., a color shading appears between the display electrode group of large number of the electric charge transmission and the display electrode group of small number of the electric charge transmission, or the coloration degree of the display gradually deteriorates as a whole.